JP2518478B2 - Infrasound detector - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the propagation of air vibrations having a frequency lower than the audible range, which occurs when a closed space defined in a house, a car, a safe, etc. is suddenly opened. The present invention relates to a low frequency sound detector suitable for detecting an elastic wave (hereinafter referred to as low frequency sound) which is a medium.

[0002]

2. Description of the Related Art A microphone is used for detecting so-called audible sound that can be heard by human ears among elastic waves propagating in air as a medium, and ultrasonic detection for detecting ultrasonic waves within about 20 kHz. Vessels are used. FIG. 5 shows the structure of a ceramic microphone, which is an example of the conventional microphone. The ceramic microphone is supported in a cantilever manner on a main body 11 and a substrate 11a provided inside the main body 11. Bimorph type piezoelectric element 12
And a vibration plate 13 stretched over the main body 11 above the piezoelectric element 12, and a pin 14 connecting the central part of the vibration plate 13 and the free end of the piezoelectric element 12. There is.

The audible sound wave is the sealed body 11
The vibrating plate 13 having a cone shape attached to the piezoelectric element 12 is vibrated via the pin 14
Is transmitted to As a result, the piezoelectric element 12 repeats flexural deformation corresponding to the frequency of the audible sound wave, and as a result, a piezoelectric output corresponding to the frequency of the audible sound wave is generated in the piezoelectric element 12.

[0004]

However, although such a conventional ceramic type microphone has some sensitivity to an infrasound, it is an apparatus for detecting an audible sound by nature, so that it can be used in a natural environment. On the contrary, various kinds of audible sounds that exist exist as noise, which is a problem because it is not possible to selectively detect only infrasound. Therefore, for example, it is practically impossible to detect an ultra-low frequency sound (18 Hz or less) lower than the audible range that is generated when the sealed space in a house, an automobile, a safe room, or the like is broken. there were. In addition, the above-mentioned destruction of the enclosed space in the house almost always occurs sporadically, and it does not occur continuously, which means that conventional microphones or ultrasonic detectors can be used in this type of ultra-low It was difficult to use for detecting frequency sound.

In view of the above situation, the present invention provides an infrasound detector which maximizes the sensitivity particularly to infrasound and is not affected by an unnecessary signal caused by an audible sound or the like. The purpose is to do.

[0006]

The infrasound detector according to the present invention comprises a container having an opening, a partition wall dividing the interior space of the container into two parts, and a piezoelectric element attached to the partition wall. A hole is opened in the partition wall at a position corresponding to the opening, and the piezoelectric element is supported in a cantilevered manner at the peripheral portion of the hole on the partition wall so as to be arranged between the opening and the hole. The piezoelectric element is formed in a strip shape having a thickness of 300 μm or less with electrodes on both front and back surfaces, and has a flat area larger than the opening area of the opening and smaller than the area of the hole. A vent having a predetermined opening area is bored along the inner wall of the container at the joining portion of the container and the partition wall.

[0007]

According to the present invention, the super low frequency sound propagated through the opening of the container into the container causes the piezoelectric element to vibrate corresponding to the frequency, and this vibration causes the vibration in the piezoelectric element. The generated piezoelectric output is sent to the amplifier circuit via the electrode and is amplified. In this case, in order to maximize the sensitivity of the piezoelectric element, it is necessary to maximize the deflection of the piezoelectric element when the super low frequency sound reaches the element surface of the piezoelectric element.

According to the present invention, the strip-shaped piezoelectric element is cantilevered at one end of the piezoelectric element at a proper position in the peripheral edge of the hole of the partition wall so that the free end of the piezoelectric element can be freely bent. Therefore, a large piezoelectric output can be obtained. The flexural deformation of the piezoelectric element becomes easier as the thickness of the piezoelectric element becomes thinner, so that the sensitivity at that time is improved, but on the other hand, the mechanical strength is weakened and it becomes difficult to form and process the piezoelectric element. The piezoelectric element needs to have at least a certain thickness or more. This thickness cannot be uniformly specified because the material and size of the piezoelectric element are different, but the upper limit of the thickness that obtains the sensitivity required for detecting infrasound is about 300 μm, and especially 100
About μm is preferable.

The thickness of the piezoelectric element is set to be considerably smaller than that of the piezoelectric element (12) used in the microphone for detecting the audible sound. The thin strip shape of the piezoelectric element improves the sensitivity by directly receiving the infrasound by the piezoelectric element itself, and at the same time completely cuts off the audible sound that causes noise. This is because. Although the ceramic piezoelectric element itself has a thickness of about 100 μm in the past, in practice, the ceramic piezoelectric element is always lined with a metal plate which also serves as an electrode. In some cases, it does not have a strip configuration. In a piezoelectric element backed by such a metal plate, the total thickness of the element usually reaches several hundreds of μm.

Further, another method for maximizing the deflection of the piezoelectric element is to make the pressure wave of the infrasound to be detected reach only one surface side of the piezoelectric element. That is, similar to the above-described operating principle of the microphone, a so-called pressure-type sound receiving system is used. However, if the pressure waves of infrasound arrive at both sides of the piezoelectric element at the same time, this is done. This is because the forces acting on the bending deformation of the piezoelectric element due to these pressure waves cancel each other out, and the bending deformation of the piezoelectric element does not occur in the end.
Therefore, according to the present invention, since the piezoelectric element is arranged at the corresponding position of the hole of the partition wall so as to divide the internal space of the container together with the partition wall, only one surface side of the piezoelectric element is provided in the opening of the container. They face each other so that they reach only one surface side where a pressure wave of infrasound is applied.

By the way, when the piezoelectric elements are arranged at the corresponding positions of the holes of the partition wall as described above, if such a structure is adopted, it becomes difficult to detect the infrasound in particular as it is. That is, in the arrangement structure of the piezoelectric element, the surface side opposite to the one surface of the piezoelectric element facing the opening of the container, that is, one of the two internal spaces of the container forms a closed space behind the piezoelectric element. To configure
In particular, in the case of so-called infrasound that has a long period, that is, a so-called broad signal, a kind of damping effect occurs and the cycle of the response output is further lengthened. There is a problem that it becomes impossible to distinguish from noise.

According to the present invention, as described above, by forming the vent hole having a predetermined opening area along the inner wall of the container at the connecting portion of the container and the partition wall, a sealed space is formed behind the piezoelectric element. An open space is formed so that it does not occur, ie behind the piezoelectric element. As a result, the piezo-electric element is bent and displaced in one direction by the direct hit of a single infrasound, and then the piezo-electric element itself is displaced in the opposite direction to restore from the flexural deformation to reach the equilibrium position. This is to enable stable damping vibration. The effect of the vent hole can be most effectively enhanced by setting the gap between the piezoelectric element and the hole formed in the partition wall to the minimum necessary size that allows the bending deformation of the piezoelectric element. .

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an infrasound detector according to the present invention will be described below with reference to FIGS. In the figure, 1
Is a container having an opening 1a, 2 is a partition that divides the internal space of the container 1 into an upper chamber 3 and a lower chamber 4, 5 is a hole formed in the partition 2 at a position corresponding to the opening 1a, and 6 is The piezoelectric element is a unimorph type piezoelectric element, which is supported on the partition wall 2 in a cantilever manner at a proper position on the periphery of the hole 5.

The container 1 has, for example, an inner diameter of 8 mm and a height of 4.
In a metal container having a size of 5 mm, the diameter of the opening 1a formed in the ceiling of the container is 1.0 as shown in FIG.
It is set to mm. The partition wall 2 has a thickness of 1 mm.
Of resin, and is installed and fixed at a height of 2 mm from the bottom surface of the lower chamber 4.

The hole 5 is provided in the central portion of the partition wall 2, and is a square hole of 2.2 × 2.2 mm □ in this example.
The piezoelectric element 6 is made of PZT composition, is formed in a strip shape with a thickness of 300 μm or less, and has a size of 2.0 × 3.0 m.
It has a size of m and has a size of 1.6 × 2.4 on both front and back sides.
The silver electrodes 6a and 6b of mm are attached and formed by a method such as vapor deposition. The piezoelectric element 6 which is cantilevered as described above has a length of its free end of about 2.1 mm and is projected on the partition wall 2 by epoxy resin or the like so as to project into the area of the hole 5. It is fixed by adhesion. The dimensions of the hole 5 and the piezoelectric element 6 are set as described above, that is, the hole 5 is slightly larger than the shape of the piezoelectric element 6, that is, the piezoelectric element 6
Is arranged inside the hole 5, so that the piezoelectric element 6 can be freely displaced when it is flexibly deformed. The two conductive wires 7 connected to the piezoelectric element 6 are
It is taken out to the outside of the container 1 in an airtight state and coupled to an amplifier having an amplification factor of 80 dB via a shield wire.

A vent hole 8 having a predetermined opening area is formed along the inner wall of the container 1 at the mating portion of the container 1 and the partition wall 2. The vent hole 8 is the outer circumference of the partition wall 2. By cutting off a part of the portion (see FIG. 3A, the shape of the vent hole 8 in this case is referred to as a), or the diameter of the partition wall 2 is set to be slightly smaller than the inner diameter of the container 1. By forming a gap between the two (see FIG. 3B, the shape of the vent hole 8 in this case is referred to as “B”). When the shaped vent 8 is used, the partition wall 2 is supported by the ceiling surface or the bottom surface of the container 1 through the columns. In this embodiment, various shapes and dimensions of the vent holes 8 were set for the test. Table 1 below shows the set values of the dimensions and the like for each test. (Table 1)

In Table 1, the test No. 4 and test NO. 5 shows a comparative example in which the vent hole 8 is not provided. In case of 4, hole 5 has 3.2
It is a square hole of 3.2 mm square. In addition, the test NO. 8 is provided with a vent hole 8, but the thickness of the piezoelectric element 6 is 40
This is a reference example set to 0 μm.

Here, FIG. 4 shows the frequency characteristics of the amplifier coupled to the piezoelectric element 6. When detecting an infrasound generated by opening and closing a closed space of this kind, its signal output is shown. The value must be 1 Vp or more so that it can be clearly distinguished from noise.

Next, a concrete test conducted by using the infrasound detector configured as described above and the test result will be described. First, 7x9 as the installation location of the detector
(Floor) x 2.7 (height) m The inside of the concrete-structured warehouse was selected, and 1.8 was installed in a suitable place such as the side wall of the warehouse.
It was carried out by opening and closing a door having a size of 0.9 m at an opening and closing speed of 0.2 m / s within an opening and closing angle of 45 °. In this case, music with a volume of 35 phons was simultaneously played as a noise source in the warehouse, and an electret microphone (WM manufactured by Matsushita Electric Industrial Co., Ltd.), which is an example of the macrophone described as the prior art, was performed under such test conditions. -063) was juxtaposed and the same test was performed.

The test results of such a test are also shown in Table 1 as the signal output of the piezoelectric element 6, but as is clear from Table 1, when the vent hole 8 is not provided (Test NO. 4 and Test NO. .5) is the output of the ultra-low frequency sound detector (Test Nos. 1 to 3 and Test N) according to the present invention.
O. It is less than 1/2 compared with 5, 6). In addition, especially test No. As is clear from the comparison with No. 8, it was demonstrated that the thickness of the piezoelectric element 6 should be 300 μm or less. On the other hand, in the test result of the electret microphone, only the signal corresponding to the audible sound was detected, and the infrasound could not be identified. Further, when the door opening / closing test is performed under quiet environmental conditions by stopping the flow of music as the noise source, the output of the electret microphone cannot be distinguished from the fluctuation of the bias level, Even in this case, the infrasound could not be identified.

Based on the above test results, the same test as above was further conducted in order to obtain the relationship between the thickness of the piezoelectric element 6 and the area of the vent hole 8 at which 1 Vp or 1.3 Vp is obtained as the piezoelectric output signal. In this case, except for the piezoelectric output signal value, the test NO. The test was performed under the same conditions as 1. The following Table 2 shows the relationship between the thickness of the piezoelectric element 6 and the area of the vent hole 8 obtained from the test result. The numerical values described in Table 2 above represent the area (cm ² ) of the vent hole 8.

In the above embodiment, particularly when the vent hole 8 (shape B) shown in FIG. 3 (b) is formed, a pillar or the like standing upright from the ceiling of the upper chamber 3 or the bottom of the lower chamber 4 is used. The partition wall 2 can be supported at a predetermined position in the container 1. However, the shape of the vent hole 8 is not limited to the shape a or the shape b, and various types having a predetermined vent hole area can be used. Can be selected. When the hole 5 is opened in the partition wall 2 at a position corresponding to the opening 1a of the container 1, the opening 1a
It is preferable to set the ventilation hole 8 around the partition wall 2 at a position where it cannot be seen straight through, so that the infrasound is circulated from the upper chamber 3 side to the lower chamber 4 side and the piezoelectric element 6 This is advantageous because it is possible to prevent the displacement from being offset. Even if the vent hole 8 can be seen through to some extent, in that case, there is a timing difference when the infrasound reaches the front side surface and the back side surface of the piezoelectric element 6, and therefore, the piezoelectric element 6 as described above. The displacement is compensated for by an amount that does not substantially pose a problem. Further, although the above embodiment has been described with reference to the case of the unimorph type, it goes without saying that a bimorph type formed by joining two such unimorph types back to back can be used.

[0023]

As described above, according to the ultra low frequency sound detector of the present invention, a detection circuit such as a filter is not necessary, and the detection
Achieve cost reduction and compactness of the entire dispenser
Is possible. Further, the piezoelectric element can be directly subjected to the impact of the infrasound that flows from the opening of the container and can be displaced to the maximum extent, and a large piezoelectric output can be obtained from the piezoelectric element. On the other hand, since it has no sensitivity to audible sound, it has an excellent advantage that it has sharp selectivity only to infrasound. In addition, the vent for air escape provided around the partition in the container can suppress the damping effect on the piezoelectric element, thereby improving the response characteristic of the piezoelectric element and improving its sensitivity. be able to.

[Brief description of drawings]

FIG. 1 is a plan view of an embodiment of an infrasound detector according to the present invention.

FIG. 2 is a vertical sectional view of the infra-sound detector of the present invention.

FIG. 3 is a plan view showing a configuration example of a partition wall provided in the container and a ventilation port provided around the partition wall of the infra-sound detector of the present invention.

FIG. 4 is a graph showing frequency characteristics of an amplifier coupled to a piezoelectric element of the above-mentioned infrasound detector.

FIG. 5 is a vertical cross-sectional view of a conventional ceramic microphone.

[Explanation of Codes] 1 Container 2 Partition 3 Upper chamber 4 Lower chamber 5 Hole 6 Piezoelectric element 7 Conductor 8 Vent

Claims (1)

(57) [Claims] 1. A very low frequency sound detector comprising a container having an opening, a partition dividing the interior space of the container into two parts, and a piezoelectric element attached to the partition, wherein the partition is provided at a position corresponding to the opening. A hole is opened, and the piezoelectric element is supported in a cantilever manner on the partition wall at a proper position on the peripheral edge of the hole so as to be arranged between the opening and the hole, and the piezoelectric element has electrodes on both front and back surfaces thereof. Is formed in a strip shape with a thickness of 300 μm or less, and has a flat area larger than the opening area of the opening and smaller than the area of the hole, and in the connecting portion of the container and the partition wall. An ultra-low frequency sound detector characterized in that a vent having a predetermined opening area is formed along the inner wall of the container.